fix(session): forward session resolution errors as StreamEvent

[Copilot]

When resolve_session() raises (e.g. botocore.exceptions.TokenRetrievalError from expired AWS SSO credentials), the exception previously propagated unhandled out of the invoke() generator, ending the stream abruptly with no error event emitted to the client.

Changes

• app.py — wrap resolve_session() in try/except Exception; on failure log via logger.exception() and yield an error StreamEvent before returning. app.state.session is only updated on success.

try:
    session = resolve_session(app.state.app_config, app.state.infra, session_id)
except Exception as exc:
    logger.exception("session_id=<%s> | session resolution failed", session_id)
    yield error_event(str(exc)).asdict()
    return
app.state.session = session

• test_app_invoke.py — adds test_yields_error_event_when_resolve_session_raises to assert that a single error event is emitted and app.state.session remains None when resolution fails.

[galuszkm]

lgtm